Java Cheat Sheet — Complete Java Syntax, Data Types, Loops, and OOP Concepts for Beginners

Java Cheat Sheet — Complete Java Syntax, Classes, and Examples

Java is a powerful, object-oriented programming language widely used for developing web, desktop, and mobile applications. This Java Cheat Sheet provides a quick reference for syntax, keywords, data types, and essential programming structures — all in one place.

What is Java?

Java is a platform-independent, high-level language developed by James Gosling at Sun Microsystems (now owned by Oracle). It follows the “Write Once, Run Anywhere (WORA)” principle, meaning compiled Java code can run on any platform that supports Java.

Basic Java Syntax

Every Java program must have a class and a main() method — the entry point of the program.

Example:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, Java!");
    }
}

Explanation:

• public class HelloWorld → Defines a class.
• public static void main(String[] args) → Main method, execution starts here.
• System.out.println() → Prints output to console.

Java Data Types and Variables

Java Operators

Java Conditional Statements

Example: if, else if, else

int marks = 85;

if (marks >= 90) {
    System.out.println("Grade A");
} else if (marks >= 75) {
    System.out.println("Grade B");
} else {
    System.out.println("Grade C");
}

Java Loops

For Loop

for (int i = 1; i <= 5; i++) {
    System.out.println("Count: " + i);
}

While Loop

int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;
}

Do-While Loop

int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);

Java Arrays

An array is a container that holds multiple values of the same type.

Example:

int[] numbers = {10, 20, 30, 40, 50};
System.out.println(numbers[2]); // Output: 30

Java Methods

Methods allow reusability and organization of code.

Example:

public class Calculator {
    static int add(int a, int b) {
        return a + b;
    }

    public static void main(String[] args) {
        System.out.println(add(5, 10)); // Output: 15
    }
}

Java Object-Oriented Programming (OOPs)

Java is built around OOP principles, which include:

Example of OOP in Java:

class Animal {
    void sound() { System.out.println("Animal sound"); }
}

class Dog extends Animal {
    void sound() { System.out.println("Bark"); }
}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();
        a.sound(); // Output: Bark
    }
}

Java Strings

Strings are objects that represent a sequence of characters.

Example:

String s = "Hello Java";
System.out.println(s.toUpperCase());
System.out.println(s.length());

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Java Exception Handling

Handle runtime errors gracefully using try-catch.

Example:

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Error: Division by zero");
}

Java Collections Framework

Collections store and manipulate groups of objects efficiently.

Java Input and Output

Example:

import java.util.Scanner;

public class InputExample {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter name: ");
        String name = sc.nextLine();
        System.out.println("Hello " + name);
    }
}

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FAQ — Java Cheat Sheet

Q1: What is Java used for?
Java is used to build desktop, mobile, and enterprise applications — including Android apps and web servers.

Q2: What is JDK and JRE?

• JDK (Java Development Kit): Used for developing Java applications.
• JRE (Java Runtime Environment): Used for running Java programs.

Q3: What is the difference between == and .equals() in Java?
== compares references, while .equals() compares actual string content.

Q4: Can Java run without an internet connection?
Yes, Java programs run locally using the JVM (Java Virtual Machine).

Q5: How do I handle exceptions in Java?
Use try-catch-finally blocks to catch runtime errors and handle them safely.